Internet telephony service providers (ITSPs) offer their subscribers Voice over Internet Protocol (VoIP) service with the “look and feel” of plain old telephone service (POTS). ITSPs provision their subscribers with Direct Inward Dial (DID) numbers in the standard format of conventional telephone numbers, as mandated by Recommendation E.164 of the International Telecommunications Union (ITU-T). ITSPs deploy gateways to interface between their IP networks and the public switched telephone network (PSTN), so as to enable direct dialing between PSTN users and their subscribers' IP phones.
Typically, when a subscriber of a given ITSP places an outgoing call to a destination telephone number that is known to correspond to an IP address (such as a DID belonging to the same ITSP), the ITSP routes the call to the destination over the IP network. When the destination telephone number is not recognized, however, the ITSP cannot generally determine whether the number indicates an IP phone or PSTN phone. Therefore, the ITSP must generally route the call through a gateway to the PSTN. If the destination number is a DID assigned by another ITSP, the call is then routed over the PSTN to the gateway of the destination ITSP.
The ENUM standard has been developed to provide standardized mapping between E.164 telephone numbers and Internet uniform resource identifiers (URIs) using the well-known Domain Name System (DNS). ENUM is defined by Faltstrum in Request for Comments (RFC) 2916 of the Internet Engineering Task Force (IETF), entitled “E.164 Number and DNS” (2000). Using ENUM, international telephone numbers are stored in the centralized DNS database in the e164.arpa domain and may be accessed using standard queries and other protocol features of DNS. DNS is described in detail by Mockapetris in IETF RFC 1034, entitled “Domain Names—Concepts and Facilities” (1987), and in IETF RFC 1035, entitled “Domain Names—Implementation and Specification” (1987). The above RFC documents are available at www.ietf.org and are incorporated herein by reference.